Automobile scrap is used in many foundries as a source of iron for cast iron. With the increasing use of aluminum in automobiles, the aluminum level in the automobile scrap is significantly higher than in years past. Moreover, it has been found that the addition of aluminum to a coupla charge may reduce coke consumption and silicon loss as well as increase the melt temperature and reduce the sulfur content of the molten cast iron produced therefrom. This iron contains higher then normal deleterious amounts of aluminum. Regardless of the source of aluminum, the aluminum level in molten cast iron may easily reach a level where harmful effects are experienced. For example, the literature indicates that aluminum concentrations greater than 0.01% by weight may cause pinholes in the castings. In addition to the pinholes, the presence of aluminum causes excess dross formation due to continuous oxidation of aluminum to aluminum oxide which all too often becomes entrapped in the melt which, in turn, introduces inclusions in the castings. Moreover, excess dross formation creates metal handling problems and increases the metal loss.
Heretofore, attempts have been made to oxidize the aluminum by bubbling air and/or oxygen through the melt or by using a high velocity lance positioned above the melt surface and projecting the oxygen/air onto the surface. Moreover, solid reagents such as iron ore, ferric oxide, sodium sulfate, or manganese oxide have also been used to oxidize the aluminum. Unfortunately, such oxidizing agents also react with any carbon and silicon present in the melt and thereby reduce their concentrations. The sodium sulfate additionally contaminates the melt with sulfur and produces sodium vapor. Chlorine and molten manganese chloride flux have also been reported as candidates for removing aluminum without affecting the silicon and carbon content of the melt. Chlorine unfortunately produces a considerable amount of iron chloride fume which causes a pollution problem. Dolomitic limestone additions have also been proposed, but this results in oxidation of carbon, absorption of heat from the melt incident to the decomposition of the carbonates, and formation of considerable MgO and CaO dust aggravated by the formation of CO.sub.2 gas. Manganese metal containing some nitrogen has been proposed, but is ineffective in reducing aluminum content below 0.01%. Finally, our copending U.S. patent application U.S. Ser. No. 941,287 filed Sep. 4, 1992, and now U.S. Pat. No. 5,240,673, filed concurrently herewith and assigned to the assignee of the present application, describes a process wherein the metallic contaminants react with droplets of molten SiO.sub.2 --CaF.sub.2. In that technique, bubbling nitrogen aggressively through the iron is used to break the molten SiO.sub.2 --CaF.sub.2 pool into small droplets and mix them into the iron for reaction with the undesirable metallics (e.g., aluminum).
The present invention significantly improves our earlier process by providing solid particles having the liquid flux on the surface thereof which significantly accelerates dealuminization while significantly suppressing splashing or expelling of molten metal from the reactor.
It is an object of the present invention to provide an improved process for dealuminizing molten cast iron and removing any other metallic contaminants susceptible to silica oxidation which process utilizes a molten SiO.sub.2 -based flux disposed on the surface of substantially free-flowing particles of solid SiO.sub.2 which carry the flux into the molten iron for highly effective reaction with the aluminum et al therein. This and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows.
The present invention contemplates an improved process for removing aluminum from molten cast iron, which process is simple, pollution-free, quick, does not deplete the molten cast iron of its carbon, enriches the iron with silicon, suppresses iron expulsion from the reactor, and provides highly reactive particles of controlled size which are significantly smaller and hence more reactive than the droplets which were characteristic of our earlier work, U.S. Pat. No. 5,240,673. In this regard, relatively large particles of SiO.sub.2 are coated with molten SiO.sub.2 --CaF.sub.2 such that the particles do not become agglomerated and serve as carriers for carrying the molten flux into the iron melt.
While the present invention is most particularly applicable to the removal of aluminum, other silica-oxidizable metals (e.g., certain Periodic Table Group IIIA, IVA and VA metals such as cerium) are also removed by the process of this invention. The present invention contemplates a method for substantially dealuminizing aluminum-containing cast iron, wherein: the iron is heated to a temperature sufficient to keep it molten throughout the period of treatment; a mass of solid, free-flowing flux particles comprising SiO.sub.2 and about 1% to about 5% by weight calcium fluoride is disposed atop the molten iron and heated by the underlying melt to melt a small portion of the flux. Once heated, the particles remain free-flowing and each comprise a core of solid silica having a molten silica shell clinging thereto and encapsulating the core. The molten shell comprises at least about 35% by weight calcium fluoride. The molten iron and flux are stirred together in such a manner as to cause the liquid-coated particles to react with the aluminum (i.e., to form aluminum oxide) and release silicon into the melt. The dealuminizing reaction is as follows: EQU 3 SiO.sub.2 +4 Al.fwdarw.2 Al.sub.2 O.sub.3 +3 Si
A side reaction between the silica and calcium fluoride yields a small amount of calcia and silicon tetrafluoride.
In a preferred embodiment, an excess amount of the particles are floated atop the iron melt and act as a blanket for suppressing expulsion of metal from the reactor incident to the vigorous stirring thereof. In this regard, stirring is such as to cause the lower portion of the particle blanket to mix intimately with the iron while the upper portion serves to provide fresh particles to the lower circulating portion as well as to suppress the molten metal being expelled from the reactor due to the mixing for better reaction therewith. The particles preferably have a mean diameter of about 0.25 to about 2 millimeters. Smaller particles have a higher tendency to agglomerate, while larger particles are more difficult to disperse in the molten iron. Most preferably, the particles will have a mean diameter of about 1 millimeter as this size is most readily stirred into the molten iron. The molten shell around the solid core wets and adheres to these relatively large SiO.sub.2 particles sufficiently that it does not coalesce with the shells on adjacent particles. Hence, the molten flux wetted particles act essentially as solid particles, remain essentially free-flowing and serve to control the size of the molten silica reactant contacting the melt. In this regard, maintaining a solid core of silica wetted with a molten shell thereover produces a reactive surface area which is much higher than heretofore found possible by dropletizing a pool of molten silica by aggressively bubbling N.sub.2 through the metal. The higher surface area provided by the liquid coated solid core accelerates and simplifies the aluminum removal process.
The molten shell comprises a relatively small portion of the particle and depends on the CaF.sub.2 content and the temperature of the flux. Hence for example, at 5% by weight CaF.sub.2 and 1550.degree. C. the liquid content will be about 15.2% by weight whereas at 2.5% CaF and 1450.degree. C. the liquid content will be about 5.9% by weight and the balance being the solid core particles. The aluminum oxide formed by the dealuminization reaction dissolves in the molten SiO.sub.2 --CaF.sub.2 flux shell. Treatment continues for a sufficient time for the aluminum content thereof to drop to an acceptable level which is preferably below about 0.01% by weight.
The flux generally consists essentially of silicon dioxide and about 1% to about 5% by weight CaF.sub.2, and preferably about 2.5% by weight CaF.sub.2 and the balance SiO.sub.2. Above about 5% CaF.sub.2 the flux loses its free-flowing, powdery character and becomes viscous and "gloppy" as the particles begin to agglomerate. Below about 1% by weight CaF.sub.2 there is insufficient molten SiO.sub.2 formed to be effective.
The flux is floated as a blanket of free-flowing particles atop the iron. The iron is vigorously stirred so as to cause the lower portion of the blanket contiguous the iron to intimately mix with the iron and react with the contaminants (e.g., aluminum) therein. The upper portion of the blanket does not directly mix and react with the iron, but rather serves to suppress splashing of the iron from the reactor and provide fresh make-up particles for the lower portion. The molten iron is preferably stirred by bubbling nitrogen up from the bottom thereof through porous ceramic plugs which are placed in the floor of the reaction vessel. Other known stirring techniques capable of mixing the silica particles into the molten iron are also acceptable.